It is known to utilize lasers as an assistance element in combination with light curtains, light gates, and light screens. The lasers can be applied either to the transmitter or the receiver strip of the screens or curtains. One known arrangement is powered by batteries and, at a distance of about 5 m, generates a light spot having a diameter of less than 10 mm. This alignment assistance device can be shifted along the strip on which the light gate bar is mounted, so that the parallel relationship of the transmitter strip and the receiver strip can be tested over the entire height of the supervised, or guard field, that is, the field subject to light between the receiver and the transmitter. This assistance device has a disadvantage, in that it is available only during the time that the strips are adjusted. The adjustment, further, cannot test at the same time for all the possible degrees of movement of the strips with respect to each other, nor can it test the mechanical relative adjustment. Once the light curtain, after adjustment, is installed, subsequent testing of the relative alignment of the receiver and transmitter strip is frequently difficult, or impossible because the bars may not be generally readily accessible. The necessary space to add the alignment devices also frequently is not available. If the alignment device is shifted along the respective receiver or transmitter strip, the receiving and transmitting functions of the light curtain must be disconnected.
U.S. Pat. No. 3,752,978, Kahl et al., describes a photoelectric intrusion detector in form of a light gate, which has an infrared transmitter and a respective receiver. Transmitter and receivers are located on respective separate attachment or support bars. To reduce the constructional depth of the light gate, a mirror is provided in each of the respective strips, which deflects a testing beam at a right angle. To initially adjust the mirror, a separate light source having visible light is used, which can be secured to the strips. The alignment of one mirror on one strip is so carried out that the light source on the other, opposite strip is mounted ahead of the other mirror, and the first mirror is so adjusted until the light beam of the light source impinges on the transmitter, or the receiver, respectively. This adjustment system, in which a working beam is adjusted between the transmitter and the receiver during installation cannot be used with a light curtain which has a multiplicity of transmitting units and receivers to provide a supervised screen or curtain, rather than for only a single beam.
Integrating an alignment assistance device with the strips or bars which retain the transmitters and receivers generally has been considered to be undesirable, because alignment can be done only at one point. In actual practice, the first mentioned alignment assistance device which can test the protected field over its entire extent is preferred, although this device is not integral with the light curtain itself. It is, further, well known that the laser sources which are generally available on the market have a substantially shorter lifetime than the optoelectronic elements used to provide the light curtain itself, and to check the integrity of the light curtain. If the alignment assistance device were to be permanently in operation during the lifetime of the optoelectronic elements, it would have to be replaced much more frequently than the optoelectronic elements themselves. Thus, integration of laser alignment devices into light gates has been generally, found to be, undesirable.
U.S. Pat. No. 5,302,942, Blau, describes a light curtain in which each receiver is coupled with a light emitting diode (LED) radiating visible light. If a receiver element received a signal a below a predetermined threshold, the diode lights, so that the operator can readily check the condition of the supervised, or guard zone. According to this disclosure, the LEDs can also be used for respective alignment of the transmitter and receiver strips or bars, by so aligning the bars with respect to each other that all LEDs respond. The disadvantage of such adjustment, however, is that only correct alignment of the transmitter bar with respect to the receiver bar can be tested, but not reasons for possible misalignment.